The through weld by using high-density energy beams, such as a laser beam, an electron beam et al., generates little thermal deformation and less segregation of the fused metal due to quick cooling for solidification at the welding sites, therefore it has been used for the stack welding and butt welding of steel materials especially thin steel sheets.
FIG. 8A shows a cross sectional view of the conventional welded joint of aluminum and aluminum alloy base metals. FIG. 8B shows a cross sectional view of the welding site of the conventional through weld of rolling steel plate (called as “SP”, hereinafter). FIG. 8C shows a cross section view of the through weld of an aluminum base metal by using MIG (Metal Inert Gas) weld method as given as the reference.
As shown in FIG. 8A and FIG. 8B, when through weld is carried out for an aluminum metal base and an SP metal base, a key hole (a molten pool or a keyhole-shaped fusion trace) is formed. Since the solidification speed of SP is slower than that of the aluminum and the viscosity of the melted SP is larger than that of aluminum, the hang down of beads is not made and the surface of a submerged-scar c2 becomes smooth and has no projections. As shown in FIG. 8C, for the case of the through weld of aluminum base metal by using MIG weld method, the molten pool d made by arc A becomes thermally conductive and no hang down of beads is made even though the viscosity of melted aluminum base metal is low and then the submerged-scar 3 has a smooth bead surface.
However, for the through weld of aluminum base metal by using a laser beam a, a hang down of beads is easily made when a thermal heat input per unit time is large. This results into forming of a needle-like projection formed at the submerged-scar.
It can be assumed that the shield gas h blown to the welding site g from a torch, wherein the shield gas can prevent the generation of blow holes and the oxidation of melted base metals, pushes the keyhole down to the side of the submerged-scar and assists the hang down of the beads. Therefore it is expected the growth of the needle-like projection e is resulted from these phenomenological factors as pushing the keyhole, low viscosity of the melted base metal, the solidification speed of the projection e in accordance with the high-speed welding.
For this phenomenological process, when a projection e is formed at the side of the submerged-scar c1, an under-fill is generated at the laser irradiated side and the mechanical strength of the welded joint remarkably decreases. Therefore, it is quite necessary to refill the filler at the site of under-fill and remove the projection e.
It may be possible to suppress the generation of submerged-scar by decreasing the thermal heat input in the conventional through weld of aluminum base metal by using a laser beam a. However, in this method, the problem that the laser beam tends to be reflected and the continuous welding may cease in sites may happen.
The forming of projection e can be prevented when a thick welding base metal is used at the side of the submerged-scar and the minimum spot size of the laser beam is set to locate far from the side of the submerged-scar to be. However it is necessary to select thick welded base metals for this method, therefore it is a problem that the range of application of this method is limited
Taking two intuitive properties; that no projections are formed for the through weld by means of MIG weld or through weld of an SP base metal by means of high-density energy beam and that the hang down of beads is generated and the projection is formed at the site of the merged-scar when the thermal heat input is increased for the through weld of aluminum base metals by using a laser beam a., it can be expected that the hang down of beads and the resultant projection forming can be prevented if the molten pool can be pushed from the side or the submerged-scar to the laser irradiated side.
An invention related to this technology that a backing plate is used for the back side of welding base metal in order to prevent the hang down of the beads and the forming of the projection for aluminum through weld by using laser has been proposed.    Reference 1    JP-A-05-228663
However, even the hang down of beads and the forming of the projection can be prevented by using the backing plate as described in this invention, it may happen that the backing plate and the welding base metal are fused to join each other. For this case, it is necessary to rework for the post process as stripping off the projections and surface polishing of the welding sites and there is a problem of additional time-consuming works.
This invention has been proposed for solving the above problem and is subject to the preventing the forming of projections by pushing the molten pool in an inert gas ambience for aluminum through weld using a laser beam.